An optoelectronic mixer or demodulator is known from German patent publication 196 43 287. In the mixer disclosed here, an avalanche photodiode (AFD) converts a high frequency amplitude modulated light signal into free charge carriers. The avalanche photodiode is connected to a reference frequency generator in order to convert the high frequency light signal with the suitable mixer frequency or reference frequency into a low frequency range.
The avalanche photodiode serves as a light sensor in this known optoelectronic mixer and must be biased in the non-conducting direction with a high voltage. The avalanche photodiode has an inherent noise which is greater than the photon noise of the light signal and has a high level of amplification which, however, is very sensitive to high voltage and temperature. For these reasons, a relatively complex calibration device is required when the known mixer described in German patent publication 196 43 287 is used in a distance measuring apparatus.
The electrooptical mixer known from U.S. Pat. No. 5,721,424 has an avalanche photodiode as a light sensor. In this electrooptical mixer, temperature dependency, non-linear AFD characteristic line and inherent noise of the AFD considerably affect the mixing efficiency.
In view of the above, it is an object of the invention to provide an optoelectronic mixer which offers a good mixing efficiency while at the same time providing reduced circuit complexity and reduced complexity with respect to its assembly.
The optoelectronic mixer of the invention is for demodulating a high-frequency light signal amplitude modulated at a signal frequency (fsig). The optoelectronic mixer includes: a light sensor for receiving and converting the light signal into free charge carriers; the light sensor having at least two individually drivable sensor electrodes; a reference frequency generator for supplying an alternating-current voltage at a reference frequency (fref); and, the reference frequency generator being connected to the sensor electrodes to alternately conduct the charge carriers to the one or the other of the sensor electrodes at the reference frequency (fref).
The reference frequency generator can apply complementary alternating voltages to the at least two individually drivable sensor electrodes of the light sensor. For this reason, the charge carriers are conducted alternately to the one or the other of the two sensor electrodes at the reference clock frequency. The charge carriers are generated at the amplitude modulation clock frequency on the light sensor. This is a multiplicative mixing operation which need not be burdened with a significant inherent noise which is different than for an avalanche photodiode as a light sensor.
In accordance with an advantageous embodiment of the invention, the light sensor is a multisegment photodiode having a plurality of individual contact segments arranged at a spacing from each other. The at least two individually drivable electrodes are formed from two segments. The simplest multisegment photodiode is a difference diode having two segments mounted on a monolithic semiconductor crystal separated by a gap of several xcexcm to several 10 xcexcm. These segments are low-ohmage transparent areas and collect all charge carriers generated at the specific segment and conduct these charged carriers away as a detectable electric current via the contacts of the segments. With the invention, an optoelectronic mixer or modulator can be built with difference diodes or even quadrant diodes. The difference diodes are relatively non-critical in operation and can be obtained in large numbers and variety. In addition, multisegment photodiodes are effective light sensors already at a low biasing voltage, for example, 5 Volts. For this reason, the complexity of the circuit of the mixer according to the invention is considerably reduced compared to the state of the art.
It has been surprisingly determined that the mixing operation takes place only in the region between the segments of a multisegment photodiode. For this reason, an especially high mixing efficiency results when an optical imaging device images the light signal into a region lying between two segments of a multisegment photodiode and the two sensor electrodes are formed by the segments next to this region. The light of this light signal, which falls into the gap or the region between the segments of a multisegment photodiode, is unexpectedly converted into charge carriers. These photoelectrons or holes are negligible for most applications and are generated in the gap and migrate to the two neighboring segments. In the event that the two segments are at the same blocking voltage, which corresponds to the usual application of a multisegment photodiode, the charge carriers, which are generated in the gap, distribute to the two neighboring segments. The distribution ratio could be dependent upon the position of the light spot in the gap. If now two neighboring segments are placed at blocking voltages of different magnitude, then the electrons (holes), which are generated in the gap, preferably migrate to the more positive (more negative) electrode.
In a further embodiment, the individual segments of the multisegment photodiode are covered so as to be impermeable to light. In this way, the surrounding light or ambient light, which could lead to additional photon noise and to overdriving, is greatly attenuated.
Surprisingly, it has been determined that the electrodes to which the reference frequency is applied, should not themselves contribute to the conversion of light into the charge carriers and that, for a multisegment photodiode as a light sensor, the gap region between the segments (which is usually deemed to be negligible), is essential for the function as an optoelectronic mixer. The light sensitivity of the segments themselves is to be suppressed as completely as possible.
In the event that the light sensor is coated with a spectral filter adapted to the wavelength range of the light signal carrier wave, disturbance light sources are suppressed with even greater efficiency. In this context, it is noted that under xe2x80x9clightxe2x80x9d herein each signal carrier is understood for which there is a suitable sensor. The invention is therefore in no way limited to the spectral range of the visible light.
In a further embodiment, the reference frequency generator is connected via very small capacitors to the sensor electrodes. Especially for a multisegment photodiode as light sensor, the reference frequency can be effectively coupled into the light sensor while saving current.
According to a further advantageous embodiment, the segments of a multisegment photodiode are biased in the blocking direction via high-ohmage resistors. This is so, because at low mixing frequencies, which arise at a low frequency spacing of the signal frequency to the reference frequency, the voltage signals can be evaluated very efficiently by the corresponding high-ohmage amplifier circuits and have low noise.
When high-impedance amplifiers are connected between the sensor electrodes and an evaluation unit for buffering and lowpass filtering of the voltage signals coming from the sensor electrodes, the high frequency signals can be separated in a manner known per se from the low frequency signals applied for the evaluation.
In accordance with another viewpoint, the invention relates to the use of an optoelectronic mixer of the invention in a distance measuring apparatus having a light transmitter transmitting the amplitude modulated light signal to the object to the measured. The light sensor detects the reflected light signal coming back from the object and an evaluation unit determines the phase shift between the transmitted and the returning light signal. The distance measuring apparatus provided in this way can be easily evaluated even for low light power, that is, a great distance of the object and/or low energy consumption and with high amplitude modulation frequency, that is, a high distance measuring accuracy. This is so because, with the optoelectronic mixer of the invention, the conversion into a low frequency range is carried out at a location in the signal path where no wideband electrical amplification has yet taken place.